1. Field of the Invention
The present invention relates to a semiconductor laser and a method of manufacturing such a semiconductor laser, and more particularly to an SDH (Separated Double Hetero junction) semiconductor laser and a method of manufacturing such an SDH semiconductor laser.
2. Description of the Related Art
One SDH semiconductor laser having a low threshold current Ith and an active layer defined by being separated from other layers by a particular crystal that is produced upon epitaxial growth is proposed in Japanese laid-open patent publication No. 61-183987, for example.
Another SDH semiconductor laser is also proposed in Japanese laid-open patent publication No. 2-174287.
FIG. 1 of the accompanying drawings schematically shows such a proposed SDH semiconductor laser. As shown in FIG. 1, the SDH semiconductor laser has a GaAs substrate 2 which is (100) oriented and has a stripe ridge 1 disposed on its (100) principal face and extending along a &lt;011&gt; direction, i.e., along an inverse mesa structure. The SDH semiconductor layer 4, a second cladding layer 32, and a current block layer 5 that are successively deposited by way of epitaxial growth on the GaAs substrate 2 including the stripe ridge 1. On the stripe ridge 1, there is formed a semiconductor stripe region 6 of triangular cross section that is sandwiched between (111) B crystal faces and dislocated from other regions. The active layer 4 in the semiconductor stripe region 6 is separate from the active layer 4 in the other regions, and the current block layer 5 is located on opposite sides of the semiconductor strip region 6 and held in contact with opposite sides of the active layer 4 in the semiconductor stripe region 6. The current block layer 5 is positioned near the vertex of the triangular semiconductor stripe region 6.
The SDH semiconductor laser is fabricated by an epitaxial growth process according to methyl MOCVD (metal organic chemical vapor deposition). The SDH semiconductor laser is fabricated utilizing such a feature of the methyl MOCVD that the rate of epitaxial growth on the (111) B crystal faces is much higher than other growth rates.
More specifically, the stripe ridge 1 shown in FIG. 1 has the (100) face on its upper surface and its opposite side edges extending along the &lt;011&gt; direction. As the epitaxial growth of the layers shown in FIG. 1 progresses, therefore, (111) faces are developed from the opposite side edges of the stripe ridge 1 upon epitaxial growth thereon. Once the (111) B faces are produced, any crystal growth is substantially ceased with respect to these faces, and crystal growth subsequently occurs from the upper surface of the stripe ridge 1 and from within grooves on the opposite sides of the stripe ridge 1. Accordingly, dislocations take place at the (111) B faces on the stripe ridge 1, producing the semiconductor stripe region 6 of triangular cross section sandwiched between the (111) B regions on the stripe ridge 1.
By selecting a width for the semiconductor stripe region 6, i.e., a width for the stripe ridge 1, and thicknesses for the various layers, a limited narrow active layer 4 is formed in the semiconductor stripe region 6 and sandwiched by the current block layer 5 for thereby limiting the flow of a current into the active layer 4, resulting in a low threshold current Ith.
Then, a third cladding layer 33 and a cap layer 7 are successively deposited over the semiconductor stripe region 6 of triangular cross section by way of epitaxial growth.
However, the semiconductor surface of the semiconductor stripe region 6 often suffers surface irregularities due to hillocks which tend to increase the threshold current Ith or irregular current characteristics.